Screening of amplifying valves



May 7, 1946.

e, LIEBMANN SCREENING OF AMPLIFYING VALVES Filed Jan; 3, 1944 Q BMm/M B W ttorve y F/cz Patented May 7, 1946 SCREENING OF AMPLIFYING VALVES Gerhard Liebmann, Cambridge, England, assignor to Cathodeon Limited, Cambridge, England,

a British company Application January 3, 1944, Serial No. 516,886 In Great Britain January 6, 1943 19 Claims.

The present invention refers to improvements in the screening of amplifying valves. It is usually necessary to provide good screening between the input and output sections of amplifying valves to avoid feed-back and instability of operation. A very high degree of screening is required for a successful high frequency amplification. The output section is in most cases equivalent to the anode, which is separated from the input section comprising the cathode and control grid by internal shields or screens.

In certain types of modern amplifier valves the leads to input and output sections of the valve are brought out at the same end or side of the envelope. Although it is possible and customary to provide shields or screens between the input and the output electrodes of the valve structure within the valveenvelope and to continue the screening outside the valve envelope, it is found in many cases that an undesirably high capacity leading to feed-back exists between the input and output sections, even if the direct inter-electrode capacity is kept low through good screening of the electrodes within the electrode structure proper. It has been found that in many cases the relatively high value of this capacity is due to the leakage capacity between the lead-in wires where they pass through the wall of the envelope, as at these points the wires cannot be surrounded by screens.

The relatively high capacity between the leadin wires has proved particularly troublesome in the miniature type of amplifier valves using miniature button bases of moulded glass, since in such valves the separation of the lead-in wires in the moulded base is quite small. In many cases the leakage capacity, even with the best possible internal screening, is as great or even greater than the direct inter-electrode capacity within the valve structure.

The applicant has already succeeded in reducing this leakage capacity considerably by inserting a pumping tube in the centre of the moulded glass base, the lead-in wires being sealed through the base in a circular array near its periphery, and inserting through the pumping tube a wire which is connected to the internal screen. This wire provides a screening electrode between the peripheral lead-in wires which lie diametrically opposite each other or nearly so. The pumping tube may be surrounded externally by a metallic tube connected to the external screen, which tube is in many cases passing partly through the glass base, utilising the depression provided in the glass base for the purpose of inserting the pumping tube. A'similar improvement has been obtained without the pumping tube, bysealing one or more screening wires through the centre (or near it) of the base and connecting such wire or Wires to the external and internal screens.

Primary objects of the present invention are to provide a further and improved solution to the problem, and in particular, an improved screening method and means which is simple to apply and highly effective for the intended purpose, enabling the leakage capacity to be reduced to a negligibly small value.

With these objects in view, the moulded glass base or other vitreous part through which the lead-in wires or conductors are sealed has the surface thereof which is situated between those lead-in wires that desirably should be screened, provided with a coating of a good conducting material which is conductively connected, or is adapted to be so connected. to the valve screening means, so that such conductive coating forms part of and completes the screening between the input and output electrodes and leads of the valve.

The invention is applicable to various valve constructions, being primarily concerned with the treatment of thevalve base or part which carries the lead-in conductors sealed therethrough; and therefore, by way of example only, some illustrative embodiments of the invention are given in the accompanying drawing for enabling the invention to be better understood and readily carried into practice.

In the drawing:

Fig. l is a greatly enlarged view in elevation of a miniature type of high frequency valve having a miniature button type moulded glass base treated in accordance with this invention, part of the valve envelope being broken away and shown in section;

Fig. 2 is a section on line 2-2 of Fig. 1, showing a plan of the electrode arrangement;

Fig. 3 is a section on line 3-3 of Fig. 4 which shows a plan, on a still larger scale, of the base used in the valve of Fig. 1;

Fig. 5 is a section on line 55 of Fig. 6 which shows a plan of a modified base provided with a pumping tube; and

Figs. 7 and 8 show in elevation and plan respectively a conventional type of valve stem treated in accordance with the invention.

Figs. 1 and 2 illustrate a typical miniature high frequency pentode having an evacuated envelope comprising a glass vessel l0 sealed to a closure therefor in the form of a moulded glass button type base II which has a circular array of seven lead-in contact pins sealed through it near its periphery. Contained within the envelope is a coaxial electrode structure comprising a central indirectly-heated equipotential cathode 12 (Fig. 2) surrounded by a control grid I3 (these electrodes not being clearly visible in the view of Fig. 1), and surrounded, in turn, by a screen grid M, a suppressor grid I5, and an anode designated generally by I6, which comprises a pair of arcuate plates ll diametrically opposed to the cathode and connected together by a pair of welded-on arcuate bands I8, as more clearly shown in Fig. 2. The electrode structure is assembled with stiff supporting wires and mica disc spacers according to conventional technique, and also includes upper and lower metal screening discs I!) and 20 which are secured to the respective mica discs 2| and 22 and are welded to supporting wires 23, thus conductively connecting the screens together. The lower screening disc is provided with a depending flange portion 24 which is welded to a lead-in pin 25. The depending portion 24 extends partition-wise between the pin 26 and the remaining five pins collectively denoted by 21, thus separating the input and output section of the valve. At one side of the partition 24 pin 25 is connected by a lead 28 to the anode I6 and at the other side of the partition the pins 21 are connected by leads respectively to the cathode, its heater and the con trol and screen grids. The suppressor grid is conductively connected at a suitable place with the internal screening elements I9, 20, 24.

In the described valve, the internal screening separating the input and output sections of the valve is constituted by the metal shields l9 and 20, the supporting wires 23, the depending flange portion 24 and the lead-in pin 25, and by the suppressor grid l5, which are all electrically connected together and grounded by pin 25. The screen grid l4 and the cathode heater are, in this construction, arranged within the input section of the valve, and therefore in this case the only electrode contained in the output section is the anode IS.

The internal screening thus provided shields the anode and its lead from the control grid and its lead, within the valve envelope. The screening may be continued outside the valve envelope in known manner. However, as already mentioned, even with careful screening inside and outside the valve in the manner described it is found in many cases that an undesirably high capacity leading to feed-back still exists, due mainly to the leakage capacity between the leadin pins where they pass through the glass of the base ll, since at this place the screening is interrupted by the thickness of the glass. This trouble is particularly aggravated with this miniature button type of base, which may be of only about 16 mm. diameter overall, with a separation between adjacent pins of only about 2.5 mm. With pins so close together, the leakage capacity, even with the best possible internal screening, may be as great or even greater than the direct grid-to-anode capacity.

In accordance with the present invention, reduction of this leakage capacity to a negligibly small value is achieved by coating either of, but preferably both, the internal and external surfaces of the base ll between the peripherally arranged lead-in pins with a layer or coating 29 of a good conducting material which is adapted to be connected to the internal and/or external screens of the valve. Figs. 3 and 4 show the base I I treated in this manner, before mounting the electrode structure thereon and sealing to the vessel ill. The conductive coating 29 is preferably applied to both sides of the base, as shown, precaution being taken, of course, to ensure that it is kept away from the lead-in pins 25 and 21 connected to the anode and other electrodes in order to maintain a sufficient insulating margin. The periphery of the base H and an adjacent margin on each side thereof are left uncoated, as shown, for sealing the base to the vessel ill in order to avoid coating material being dissolved in the glass seal. The conventional button-type base is customarily formed with bosses 30 surrounding the lead-in pins, and in order to maintain the necessary insulating margin these bosses are left uncoated. This does not need to apply to the pin 25 connected to the internal screens, because, as the coating 29 is to be brought into conductive connection with those screens, it is most convenient for this purpose to have the coating extend over the bosses around that pin and into conductive contact with the pin, as shown. At the point of contact with the pin 25, the conductive coating is preferably reinforced, as by increasing the thickness of the coating thereat, in order to ensure a good electrical contact between the coating and pin. For the same purpose, the coating may be extended to cover the pin, as shown at 29c. Both sides of the base being treated in this manner, the pin 25 thus conductively connects the coatings on the two sides to each other and to the internal screens. Connection between the coating and the internal screens can obviously be made in other ways, for instance, by sealing a thin connecting wire or piece of metal foil to the surface of the base before the coating is applied thereto, or by means of separate spring contacts making connection with the coating.

In some cases, a graphite coating has proved useful but a metallic coating, preferably of silver, is generally preferred owing to its better conductivity. However, both graphite and metal may be used together, for instance, by coating the inside of the base with graphite and the outside with the metal. A method which has been found satisfactory and simple to apply in many practical cases, is to apply a good coating of graphite to the inside of the base before the electrode structure is mounted thereon, and to apply a metal conductive coating of the type usual in the art for metallising, to the outside of the base after the valve has been assembled and the sealing and pumping operations completed. It a graphite coating is used, contact to it is preferably made by means of a spring contact in order to ensure good electrical connection. The coating of the base may be completed before the electrode structure is assembled on it, or, alternatively, as already mentioned in connection with the graphite coating, the coating whether of graphite or metal may be applied to the inside of the base before the valve structure is mounted. and after the valve has been assembled, sealed and evacuated, the coating on the outside of the base may then be applied to complete the screening.

The conductive coating can be applied by any convenient process, as by evaporation, sputtering. electro-deposition or any chemical method of deposition. The application of the coating may be combined with the application of a coating of high conductivity to the lead-in pins. A suitable way of preparing a metallised base is to apply a suspension of silver oxide. in a suitable organic oil to the appropriate parts of the base, and heat the base in air first to a moderate temperature for drying the oil suspension and then to a higher temperature suitable to reduce the silver oxide to the metal. The base may be heated to a higher temperature, for instance, approximately 450 C., than is required for this reduction, in order to convert the formed silver deposit into a hard and strongly adherent layer. An alternative method of procedure could be to heat the base only to the lower temperature necessary to reduce the silver oxide, and to rely on the heat developed in the process of sealing the base to the valve vessel to harden the silver layer. The reduction of the silver oxide could be combined with the annealing process of the base.

The valve depicted in Fig. l is sealed off at the top after pumping. In the modification depicted in Figs. 5 and 6, the moulded glass base I H], which is similar to base I I, is provided in its centre with a pumping tube 3|, through which the valve envelope is evacuated, the tube 3| afterwards being sealed off. In these figures, the pins 250, 260, 210 and bosses 300 are similar respectively to the pins 25, 26, 21 and bosses 30 in Figs. 3 and 4, and in precisely similar manner the conductive coating 290 is applied to the inside and outside of the base I I0 around the pins, and over the pin 250, as indicated at 290a. As indicated in Fig. 5, the coating on the inside of the base extends partly into the tube 3|, and overlaps the coating on the outside of the base where it covers the surface of the depression 32 in the outside of the base in which the tube 3| is inserted. Thus,

in this modification, the conductive coating canv take the place of a screening wire inserted through the pumping tube, as mentioned earlier in this specification.

In analogous manner, the method of treatment according to the invention can be applied to other forms of glass stems or bases through which lead-in conductors are sealed. Figs. '7 and 8 illustrate, for example, how the invention can be applied to the conventional pinch type of valve stem in order to reduce the direct leakage capacity in the stem- In this case, a wire 33 is sealed through the pinch 34 between the anode and grid leads 35 and 36 to connect to the internal screens of the electrode structure, and is connected to a screen 31 fitted within the tubular part 38 of the stem between the grid and anode leads. The glass surface of the pinch 34 where the wire 33 is sealed through is provided with the conductive coating 39.

The eifectiveness of the improved screening method according to the invention is illustrated by the following measurements taken on a miniature type high frequency pentode with good internal screening, mounted on a button type miniature base as shown in Figs. 1 to 4. The direct grid-to-anode capacity of the electrode structure without base, was 0003010003 micromicrofarad. The leakage capacity between input and output leads in the base with all internal valve screens in position and before application of the conductive coating, was 0.0035i0.0003 micromicrofarad. The leakage capacity in the same base after the conductive coating was applied, was less than 0.0003 micromicrofarad, i. e. smaller than could be measured. For the completely assembled valve the feed-back capacity was reduced from 0.0065 'micromicrofarad to 0.0030 micromicrofarad, i. e. to less than half.

Although the invention has been particularly described with reference to the treatment of valve bases and stems, it will be apparent that the improved screening method can in suitable cases be applied in like manner to portions of the wall of a valve envelope through which lead-in conductors may be sealed.

I claim:

1. A valve having conductors sealed through a vitreous support and connected to respectively an input electrode and to an output electrode within the valve, and having internal screenin means disposed so as to screen said output electrode and the portion of its conductor within the valve from said input electrode and the portion of its conductor within the valve, the surface of said support between said input and output conductors having a conductive coating thereon insulated from said electrodes and conductively connected to said screening means.

2. A valve comprising a vitreous envelope containing at least a cathode, a control grid and an anode connected respectively to conductors sealed through the same portion of the envelope wall so as to be relatively adjacent one another, and screening means Within the envelope disposed so as toshield the anode and the portion of its conductor within the envelope from the grid and the portion of its conductor within the envelope, the surface of said portion of the envelope wall having a conductive coating thereon between said anode and grid conductors, but insulated therefrom, said coating being conductively connected to said screening means.

3. A valve as defined in claim 2, wherein the coated portion of the envelope wall is provided with a conductive coating both internally and externally, the internal and external coatings bothbeing conductively connected with said screening means.

4. A valve as defined in claim 2, in which the coated portion of the envelope wall is coated both internally and externally, and a conductor is sealed through said wall portion in conductive contact with both said internal and external coatings, said conductor being connected within the envelope to said screening means.

5. A moulded vitreous base for a valve, having a circular array of lead-in pins sealed therethrough near the periphery of the base and having a conductive coating applied to both sides thereof which is electrically connected to one of 4 said pins and is insulated from the remainder.

6. A moulded vitreous disc-like base for a valve, having a plurality of lead-in pins sealed therethrough in a circular array near the periphery of the base, each side of the base having a conductive coating thereon, the periphery of the base being uncoated, the coatings on both sides being in conductive contact with one and. the same pin so as to be thereby electrically connected together and being insulated from the remaining pins.

7. A base as defined in claim 6, in which the coatings on both sides of the base are of metal.

8. A base as defined in claim 6, having a graphite coating on one side thereof and a metal coating on the other.

9. A base as defined in claim 6, in which said coatings cover also the said pin to which they are connected.

10. A valve as defined in claim 2, wherein the coated portion of the envelope wall is provided with a graphite coating internally and a metal coating externally, the internal and external coatings both being conductively connected with said screening means.

11, A valve as defined in claim 2, wherein said portion of the envelope wall comprises a. moulded vitreous base through which said conductors are sealed and which is sealed to a vitreous vessel to form said envelope.

12. An electron valve comprising an envelope having a disc-like vitreous base closing one end thereof, a plurality of rigid metal contact pins sealed through said base and extending from opposite faces thereof, a conductive screening coating on and extending across at least one of said faces of said base, said coating being conductively connected to one of said pins but insulated from the remainder, an electrode assembly within the envelope and including a control grid and an anode conductively connected respectively to pins at opposite sides of said pin connected to said coating, and metallic shield members within the envelope disposed for electrostatically shielding said anode and the portion of said anode pin within the envelope from said control grid and the portion of said control grid pin within the envelope, said shield members being conductively connected to said pin connected to said coating.

13. A valve as defined in claim 12, in which said screening coating is applied to each of said faces of said base, and the coating on each face is conductively connected to the same pin.

14. A valve as defined in claim 12, in which said coating is of metal and extends over the surface of said pin connected thereto.

15. A valve as defined in claim 12, having a graphite coating on the face of said base within the envelope and a metal coating on the face of said base outside the envelope.

16. A valve as defined in claim 12, wherein said base has integral vitreous bosses through which said pins are sealed and extend, the bosses of the pins that are insulated from said coating bein free from said coating thereby to insulate such pins from aid coating.

17. An electron valve comprising an electrode assembly, an enclosing envelope for said assembly having a disc-like vitreous base and an exhaust tube extending outwardly from the center of said base, a plurality of lead-in pins including control grid and anode pins sealed through said base in a circle around said exhaust tube and projecting from opposite faces of said base, and electrostatic shielding means including a conductive coating on and extending across the face of said base within the envelope and extending into said exhaust tube on the internal surface thereof, said coating being insulated from said control grid and anode pins and being conductively connected to a third lead-in pin located between said control grid and anode pins.

18. An electron valve comprising an envelope containing an electrode assembly and closed by a disc-like vitreous base supporting said assembly, an exhaust tube extending outwardly from the center of said base, a plurality of lead-in pins including control grid and anode pins sealed through said base in a circle around said eX- haust tube and projecting from opposite faces of said base, and electrostatic shielding means including a conductive coating on and extendin across the face of said base within the envelope and extending into said exhaust tube on the internal surface thereof, and a conductive coating on and extending across the other face of said base outside the envelope and extending on the external surface of aid exhaust tube, aid coatings being insulated from said control rid and anode pins, and being conductively connected to a third lead-in pin disposed between said control grid and anode pins.

19. A valve comprising an envelope containing cooperating electrodes including a control grid and an anode, a disc-like vitreous base closing one end of said envelope in a vacuum-tight manner, said base having a depression in the center of the face thereof outside the envelope and having an exhaust tube extending outwardly from said depression, lead-in pins including control grid and anode pins sealed through said base in a circle around said depression and exhaust tube, and electrostatic shielding means comprising metal shield members within the envelope disposed for electrostatically shielding the anode,

and the portion of the anode pin within the envelope from the control grid and the portion of the control grid pin within said envelope, a conductive coating on the face of said base outside the envelope and extending over the surface of said depression around the external surface of said exhaust tube, and a conductive coating on the face of said base within the envelo e and extending into said exhaust tube on the internal surface thereof to overlap the portion of said firstmentioned coating on said depression, said coatings and said shield members being conductively connected to a common lead-in pin located between said control grid and anode pins.

GERHARD LIEBMANN. 

